3 Answers
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There many verses that use the word salvation, but they make more sense of you see salvation (check out the Greek tenses and cases of this word) as a processes that will be completed over a period of time.

I can think of the following if I may take the liberty of quoting them out of context!

Old Testament

Psalm 138:8 (NKJV)28 The Lord will perfect that which concerns me;
Your mercy, O Lord, endures forever;
Do not forsake the works of Your hands.

New Testament

John 10:28-29 (NKJV)28 And I give them eternal life, and they shall never perish; neither shall anyone snatch them out of My hand. 29 My Father, who has given them to Me, is greater than all; and no one is able to snatch them out of My Father’s hand.

9 He has saved us and called us to a holy life—not because of anything we have done but because of his own purpose and grace. This grace was given us in Christ Jesus before the beginning of time, 10 but it has now been revealed through the appearing of our Savior, Christ Jesus, who has destroyed death and has brought life and immortality to light through the gospel. 11 And of this gospel I was appointed a herald and an apostle and a teacher. 12 That is why I am suffering as I am. Yet this is no cause for shame, because I know whom I have believed, and am convinced that he is able to guard what I have entrusted to him until that day.

Here's another good one from Hebrews.

Hebrews 7:25 (NIV)
Therefore he is able to save completely those who come to God through him, because he always lives to intercede for them.

And from one of my favorite passages in scripture (read the whole context, it's great!)

I Peter 1:5 (NIV)
who through faith are shielded by God’s power until the coming of the salvation that is ready to be revealed in the last time.

The following passages speak of us being 'sealed' (i.e. a completed transaction, we now belong to God). Or to quote from Baker's Evangelical Dictionary of Biblical Theology: "A seal, in biblical times as today, is used to guarantee security or indicate ownership" (source).

Ephesians 1:14 (NIV) who is the down payment of our inheritance, until the redemption of God’s own possession, to the praise of his glory.

Ephesians 4:30 (NIV) And do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption.

2 Corinthians 1:22 (NIV) who also sealed us and gave us the Spirit in our hearts as a down payment.

There is no question that all Christians are eternally secure. The root argument for this doctrine is ‘predestination’.

If one looks at individual Bible verses on other subjects, pulls them out of context and then tries to conclude eternal security ‘yes’, or ‘no’, one never really finds any certainty. On the other hand if one just looks at the doctrine which directly speaks to the subject all confusion vanishes instantly. There is no doubt. It is as certain as the doctrine of God existing.

For example, in Romans 8:30 we see the full certainty of salvation from beginning to end. From calling to glorification in heaven:

And those whom he predestined he also called, and those whom he called he also justified, and those whom he justified he also glorified. (Rom 8:30, ESV)

Now predestination means God chose a person ‘for salvation’ (not salvation and then destruction). He did this ‘before the foundations of the world’:

even as he chose us in him before the foundation of the world, that we should be holy and blameless before him. In love he predestined us (Eph 1:4–5, ESV)

The doctrine of predestination means specific ‘individuals’ are called and that God knows them by name, calling each one. In fact every Christian are called ‘the elect.’ ‘The elect’ is not some mysterious concept but a title for any Christian.

So there is some common sense to be had here. If God chose you before your were born and called you ‘elect’ this is a ‘title of blessing.’ If God knowing you before you were born and knowing everything you will do in your life decided to given you a title that implies ‘great blessing’, God would be very much an evil liar if you ended up in hell, which is impossible. Therefore there is only one rational conclusion. All who are elect, that is Christians, are blessed, from the standpoint of eternity and nothing in time can ever possibly alter that.

Eternal security is pretty central to understanding the gospel. Those who reject the scriptures concerning the doctrine of election can still be among the elect but it can only cause much confusion in understanding the various scriptures that discuss the subject.